Transfer Port Theory

[Deleted member 12676]

Factors affecting transfer flow: Transfer area, engine displacement, crankcase pressure.
Here's the formula for flow velocity:
Velocity (m/s) = Volume flow (m3/s) / area (m2)
Air/fuel velocity through transfer ports increases with greater air/fuel volume flow thru the same port area (which happens when you modify the engine with a larger bore), and decreases with greater port area to channel the same air/fuel volume (which happens when you enlarge the transfers or add boost ports).
Here's my theory on how air/fuel velocity change is related to powerband change. First off, increasing the crankcase compression ratio (CCR) increases the pressure that exists in the crankcase right before the transfers open. More pressure increases the volume flow and thus the velocity. So there are 3 things that can increase transfer velocity; enlarging the cylinder bore, reducing the transfer port area, and increasing CCR.
Why is velocity important? More transfer speed means the air/fuel charge will reach the spark plug faster. You want the flow speed to match the type powerband you want. The scenario most likely is that the end of the transfer charge arrives at the spark plug (when it fires) at the lower rpm of the powerband, and the beginning of the charge is at the spark plug at the upper rpm of the powerband. You can slow down that arrival time by changing the transfer ports roofs to be more level. That directs the flow so that there is more of a horizontal vector and less of a vertical. More of the flow energy is dissipated in the left and right flow collision with more horizontal expansion of the combined charge.
What is interesting is that small engines need less percentage of the cylinder bore circumference than larger engines. Here's an example to prove it: A 45mm bore and stroke engine has 1590mm bore area. The circumference is 141mm which is 9% of the 1590. Whereas a 90mm bore/stroke engine has 6361mm bore area. The circumference is 283 which is 4.4% of it. So if the two engines have the same percentage of circumference for transfer port horizontal opening the 45mm bore engine has twice the flow area. If they have the same percentage of the circumference (say 50%) then that means the small bore engine needs less CCR to achieve the same transfer flow velocity. Maintaining the same CCR allows the small engine to be more adept at high rpm running than larger engines.
I have always been perplexed at why the designers of my small cylinders (48cc, 55cc, 60cc) have only one transfer on each cylinder side and utilize a low CCR. Having narrow transfers strikes a balance with the low CCR. Both can be small just because it's a small engine which means its transfers are automatically more efficient.
Boost ports
Since the flow from boost ports is mostly vertical with no collisions then it arrives at the spark plug even faster than the transfer flow. I had experimented with boost port size and determined 30% of the transfer port is ideal. How some engines get away with huge transfers is beyond me. Maybe that is just a fad, and in reality the engines could run stronger if their boost ports were narrowed.

 

[Fabian]

Jaguar, i like your theory and enjoy reading about it.

Please give us more!!!

 

[Deleted member 12676]

If the engines are "square" then the 45mm one is 72cc and the 90mm one is 573cc. Let's double check my theory:
If the large engine has 50% of the circumference for the transfers and the boost ports then the horizontal opening of them totals 141mm. That divided by the bore area equals .022 (2.2%). Applying that to the small engine (.022 x 1590) you get 35mm total horizontal opening. 35 divided by 141 = .25 so that only 25% of the circumference is needed to have the same ratio as the large engine. Now if we figure in the port height in comparison to the engine displacement: small engine 8mm port height, large engine 16mm port height. 8 x 35 = 280 square mm which divided by 72cc gives a ratio of 3.9 to 1. For the large engine 16 x 141 = 2,256 square mm which divided by 573cc gives a ratio of 3.9 to 1.

 

[Deleted member 12676]

Also there is less distance for the intake charge to travel to the spark plug with small engines, another factor that gives reason for less CCR and flatter transfer roofs.

 

[Old Bob]

Read Jennings,Gordon,Bell and a few others.

You can not simply scale down ratios from larger engines.Also you forgot to consider the effects of rpm on velocity and the time duration the ports have to accomplish their job.

Please read before drawing conclusions on your own.

 

[Deleted member 12676]

Bob, what do you think I did? get drunk and then make this **** up? I have studied my ass off for the past 1.5 years as well as done my own research because all info available is still very lacking. Why don't you fully understand what I've read and then give me a good comeback with details from your own emphasis comparing a small and a large engine? It's too easy to pretend that you are in-the-know but it's hard to prove it. Give me details, a technical example. You can't say "you can't do that" without showing why (with details).
If I am wrong I will admit it, no problem. I would like my fresh analysis to help improve the world of two stroking and if others can help me refine my ideas then that is fine. Sometimes I correct my own self. For instance, the increased velocity from increased crank pressure is most likely only significantly greater at the beginning of the port opening. It would take a direct measurement to reveal the whole truth. But the CCR doesn't just affect the transfer velocity, it also affects the intake velocity. More CCR has the effect on the intake of making its movement more "crisp" because the increased vacuum allows less lag. My next real life test will show me if it is able to affect low rpm power as well.
There are so many factors involved for an absolute complete understanding that it's mind-boggling. But I'd like us all to have a good understanding of the most basic factors and some generic rules to follow.

 

[Deleted member 12676]

I did not forget "rpm, velocity, port duration".
The background assumption between the two engines is that they both have the same peak rpm and the same port durations. (How could an honest comparison be otherwise?) If the percentage ratios between the piston area and the horizontal port open area are the same then the flow velocity is the same because the large piston travels at twice the velocity of the small piston but has twice the distance to travel between port opening and BDC. So the port duration time remains the same for both engines although the transfers are twice as high in the large engine. (everything in the large engine is 2x the small engines.) In the end the velocity would be the same because the transfers would have the same proportional area to piston displacement in both cases and the same port duration.

 

[Old Bob]

You are wrong.
You are assuming too many things, in the real world frictional loses and boundary layer effects will change the flow characteristics.

Read up on flow in a duct.Read Jennings about time areas.


I'll pit my 30+ plus years of study in addition to my dyno and flow bench experienceand building both four stroke and two stroke engine from raw material to your 1.5 years.

Trust me read up on this stuff before drawing conclusions.

 

[Deleted member 12676]

"frictional losses and boundary layer effects" is all you have? c'mon man, get real. prove to me in some kind of real example how I am wrong about the relation of transfer port widths to cylinder bore. It's just basic physics. It's the port area being a percentage of the bore area. Are you going to tell me that that is not important? You going to tell me that if a small and large engine both have accumulated widths the same percentage of the bore circumference, and that leaves the large engine with transfer areas as smaller percentage of the bore area that it don't matter? WTF? think, man, think. get out of the box (the limitation of what has already been written). Progress means building on top of the past, not being limited to it.
I have pointed out something that no one else has ever done and recorded for future generations to see and you are upset that it can't be possible that someone with so little study time under his belt could do so. Maybe you haven't seen my web site. Click on my signature link. I also have figured out completely about how expansion chambers work and how to accurately, using calculated return waves with an Excel file that took me months to make, analyze and design pipes. It's totally breakthrough. You gonna throw a fit about that to? Some people are forward thinkers and some people want to only defend the past. It's obvious which one you are.
The time-area idea for ports is obvious, again being basic physics, but the formula Jennings gave us from the Yamaha engineers was only accurate for the Grand Prix engines they were working with. If in your 30 years experience if you had ever played around with it you would know that and wouldn't now be throwing it at me as if you have some kind of valid point.
You keep telling me to read, but it don't seem like you know how, otherwise you'd know from what I've already wrote that I have read all that's available and then some. As an example, I brought the idea to this forum of intake extension for piston port intake engines, to increase low rpm power, from an obscure research paper I stumbled across. Crap, I bet you haven't read Blairs stuff. I barely made it through because everything he wrote was for engineers familiar with many complex formulas. You remind me of 2door, the dumbazz moderator on the other forum that had never even heard of using JBWeld in ports to modify them although famous 2 stroke gurus had been doing it for decades. 2door had lots of experience under his belt and thought he was at the pinnacle of knowledge whereas in reality he just knew a few things really well and had a very closed mind. Like I said, give me an argument with details. I'm waiting...

 

[Deleted member 12676]

Bob, you just shot yourself in the foot. I just looked at the classic port time-area formulas and they have the port areas in direct relation to the engine displacement. That's exactly what I am doing, and as a side issue have proven how small displacement engines don't need ports as wide (as percentage of bore circumference) as large displacement engines do. In the 3rd post I gave details proving my point. So don't throw "time-area" at me. As I said (jeesh, how many times do I have to repeat myself?) the "time" for both engines is the same, and the area (as a percentage of engine displacement which is what Jennings advocated) when it has the same percentage relation to displacement for both engines results in the small engine needing ports narrower (as percentage of bore circumference) than the large engine.
I gave details! I proved it!